The present invention relates to instruments for measuring gas density or pressure and more particularly to consumer pressure gauges, altimeters and the like.
Many different methods and devices exist for measuring gas pressure and density. Many use complex mechanical arrangements. For example, in U.S. Pat. No. 3,745,384 to Blanchard, a diaphram which has a varying resonant frequency depending upon the pressure applied is used with a feedback circuit for maintaining resonance.
Piezoelectric materials are used in a number of gauges. Pressure is applied to one side of the piezoelectric material, often a quartz crystal, causing stress in the piezoelectric material and thereby changing its electrical characteristics, which change can be measured by a variety of methods. See, for instance, U.S. Pat. No. 2,558,563 to Janssen, U.S. Pat. No. 3,479,536 to Norris, U.S. Pat. No. 3,903,733 to Murayama, et al. and U.S. Pat. No. 4,175,423 to Braun, et al. These devices use different mechanical mounting methods which can be quite complex.
Piezoelectric materials, such as quartz crystals, can also be caused to oscillate by the application of an external voltage. Pressure can be applied to deform the crystal and change the frequency of oscillation. The changed frequency of oscillation can be measured to determine variations in gas density or pressure. A difficulty with this method is the damping effect on oscillations if pressure is directly applied, and the hysteresis effects if pressure is applied to the edges of the crystal. Complex mechanical provisions to transmit force to the crystal without causing damping or hysteresis effects have been developed. See, for instance, U.S. Pat. No. 3,561,832 to Karrer, et al., U.S. Pat. No. 4,562,375 to Besson, et al. and U.S. Pat. No. 4,550,610 to EerNisse.
An alternate method shown in U.S. Pat. No. 3,879,992 to Bartera, uses a device which has at least two crystals, each of which is coated with a different type of gas absorbing coating. The different coatings will absorb different types of gases (such as H.sub.2 O or CO.sub.2). The increased mass of the crystal will vary its frequency of oscillation, with the amount of mass absorbed being proportional to the frequency of oscillation, thereby enabling the detection of a particular type of gas by monitoring the change in frequency. All of the crystals used are exposed to the same air environment with different gases being detected by the different sensitivities of the coatings on the crystals.
There is a need for a gas density transducer which is simple and economical for use in consumer items. Many of the discussed prior art devices are expensive and complex, requiring expensive electronic support equipment and thus warranting use only in complex systems such as military aircraft, oil well monitoring equipment or the like where cost is less critical, and being unpractical for consumer applications, such as portable battery powered applications in altimeters for bicycles, watches, cars, etc.